Oceanic Oxygen:

Feedback Effect on Marine Life May Affect Atmospheric Oxygen

By John Toon

Researchers have identified a simple feedback effect that may help control oxygen
levels in the earth's atmosphere by altering the productivity of marine organisms.
By adjusting the internal oceanic recycling of a key nutrient in response to changing
levels of oxygen, the feedback effect would regulate the growth of ocean plants that
produce oxygen as a by-product of photosynthesis.

Described in a paper published January 26 in the journal Science, the work
provides new information on how complex geological and biological systems work
together to maintain the planet's environment within the narrow bounds necessary to
support life. It may also provide additional clues to other complex aspects of
atmospheric chemistry.

Researchers prepare a box core sediment sampling device to make
measurements off the coast of Peru.

"There is a feedback between the use of oxygen on the land areas and the
productivity of organisms in the oceans," said Philippe Van Cappellen, assistant
professor in the School of
Earth and Atmospheric Sciences at the Georgia Institute of Technology. "We now have
a direct link between inorganic and biological processes that at first seemed to be very
disconnected."

"I first noticed that something was going on with phosphorus in sediments several
years ago during a study of an ancient sediment sequence," Ingall explained. "With
changes in water
oxygenation, major changes were always observed in the distribution of phosphorus in
the sediments."

Ingall saw similar effects in more modern sediments examined off the coast of Peru
during research conducted with Richard Jahnke of the Skidaway Institute of Oceanography
in Savannah, GA. In both ancient and modern sediments, the researchers
measured
larger amounts of phosphorus in sediments produced during conditions in which
oxygen levels in the surrounding ocean water were high. Conversely, low levels of
water oxygenation corresponded to reduced amounts of phosphorus
stored in the sediments.

"We find that when the oceans are fully oxygenated, the sediments tend to be more
effective sinks for phosphorus than when the oceanic waters are depleted in oxygen,"
explained Van Cappellen. "This is critical because the productivity of the oceans
appears to be limited by how much bioavailable phosphorus is available."

Well-known oxygen-dependent chemical processes centered on iron oxides account
for a portion of the phosphorus uptake. But the two researchers believe a biological
factor -- likely bacteria living in the ocean floor sediments -- also play a significant role.

"Iron alone cannot explain what we are seeing," Van Cappellen said. "We propose
that the bacteria living in the top layers of sediments can be efficient scavengers of
phosphorus, though this scavenging seems to be linked to the availability of oxygen.
These bacteria can switch from one type of energy cycle to another depending on
whether they have enough oxygen. Microbial physiology therefore may play
a role in how efficiently phosphorus is removed from the water column."

Such a microbial effect has been observed in freshwater lakes, and also in the
operation of wastewater treatment plants, where high levels of oxygenation result in
improved removal of phosphates, he noted.

Over geologic time, oxygen levels in ocean water are tied closely to the amount of
oxygen in the atmosphere. This proposed feedback system would therefore counter
fluctuations in atmospheric oxygen caused by changes in oxygen consumption during
weathering of minerals and old organic matter exposed on land.

Because the marine plants also remove carbon dioxide during the photosynthesis
process, the proposed feedback cycle could help researchers attempting to better
understand other aspects of global climate change.

The model advanced by Van Cappellen and Ingall may also help explain the
environmental stability that would have been required for life to thrive and evolve on
the earth during the past 500 million years. Without steady levels of atmospheric
oxygen, higher forms of life would have been unable to survive.

The work may also lead researchers to more closely examine the role of phosphorus
in the environment. Because the nutrient exists in low concentrations in sediments, it
has been difficult to measure and often ignored by scientists measuring carbon, oxygen,
sulfur and iron -- whose effects have been more completely documented.

Van Cappellen notes that the proposed feedback mechanism shows the
interdependence of seemingly-unrelated parts of the global system.

"You really cannot think about the evolution of the earth or the atmosphere
without looking at the links to biology," he said. "You must consider how the
biosphere interacts with the geosphere. I think this shows that life has developed ways
of compensating for any drastic changes in its environment."